Coccidioides immitis and C. posadasii are filamentous fungal pathogens and causal agents of Valley Fever, or coccidioidomycosis. These pathogens are increasingly responsible for significant morbidity and mortality in immunocompetent patients. Over the past several years, Valley Fever cases have increased dramatically. The genus Coccidioides was recently recognized as two species. The two species are divided into at least two populations: northern and southern California for C. immitis, and Arizona/Mexico and Texas/South America for C. posadasii. The nature of the species and population boundaries are of particular interest due to the recent discovery of hybridization and introgression within Coccidioides. Multiple putative hybridization regions were located in several genomes analyzed. However, one particular region of introgression represents a unique opportunity to assess genes that are highly likely to be relevant for species-specific virulence and adaptation to mammalian hosts. This region has a shared recombination point flanking a metalloproteinase gene MEP4, genes that are highly expressed in the parasitic phase, as well as genes of unknown function. Importantly, evolutionary selection has preserved this region in multiple strains of C. immitis further emphasizing the likely role in virulence of these genes. Variation among strains for virulence in murine models of coccidioidomycosis has been observed, but has not been tested in the context of the newly discovered species or with a focused targeted underlying genetic mechanism hypothesis to test. Regardless of whether these genes are responsible for the increased infection rate observed in Arizona, which is possible, functional characterization of these genes will significantly expand our knowledge of Coccidioides pathogenesis mechanisms which may lead to novel vaccine candidates and/or new therapeutic options. In this proposal, the function and role of the genes and proteins in this introgressed region will be analyzed to determine their importance in fungal virulence and host interactions.